Efforts to rescue survivors trapped inside the Sasago Tunnel were hindered by heavy smoke after one vehicle caught fire.
Rescuers temporarily suspended work because of fears of a further collapse at one stage.
They were attempting to reach a number of vehicles believed to be buried in the rubble, including a lorry whose driver was trapped inside and had called his company for help.
“I could hear voices of people calling for help, but the fire was just too strong,” said a woman interviewed by public broadcaster NHK after she escaped.
The Fire and Disaster Management Agency issued a statement saying five people were confirmed to have been in a car that caught fire inside the tunnel, and at least one other was in a lorry. However, officials said they could not confirm the exact number of people believed dead.
Executives for the private company that runs the Central Japan Expressway said it was investigating why the concrete panels had given way. A safety check of the three-mile tunnel’s roof in September and October found nothing amiss, they said.
The tunnel, which opened in 1977, is one of many in mountainous Japan. The location of the collapse, about a mile inside the tunnel, was complicating rescue efforts, reports said.
Police vehicles, fire trucks and ambulances were massed outside the tunnel’s entrance.
The tunnel, one of the longest in Japan, is about 50 miles west of Tokyo and is one of Japan’s most important traffic arteries.
Four workers are missing and presumed drowned and another four seriously injured after an under-construction dam partially collapsed in western Cambodia, officials said.
Local police chief, Major Theang Leng, said the hydroelectric dam on the Atay river appeared to have been holding too much water and then started leaking.